1. Field of the Invention
The present invention relates to an image pickup apparatus and an exposure control method preferable for use in, for example, a digital camera.
2. Description of the Related Art
Conventionally, an image pickup apparatus such as a silver-halide camera, a digital camera, or a video camera has an automatic exposure (AE) function for determining exposure corresponding to the brightness of a subject and controlling the shutter speed, the aperture and the like. The exposure control action involves a known photometry method for measuring the brightness of a subject, such as average photometry for measuring the brightness throughout the subject, center-weighted photometry for weighting at the center area, multi-photometry in which photometry regions are provided at a plurality of areas on a subject, and spot photometry for measuring the brightness at a specific part of a subject. In common, a digital camera measures the brightness (luminance) of a subject through center-weighted photometry when a taken image of the subject is displayed on its electronic view finder such as a liquid crystal monitor (at the monitor through mode).
In exposure control through center-weighted photometry, for example, an image of a subject is divided into 64 blocks, 8 vertically by 8 horizontally, as shown in FIG. 12A. In addition, the blocks are classified into blocks A in the center area (indicated by the bold line) and other blocks B, C, and D in the peripheral area as shown in FIG. 12B. Further, the blocks A, B, C, and D are weighted 16 times, 8 times, 4 times, and one time, respectively, as the weighting becomes greater from the peripheral area to the center area. Then, an average value of the luminance (Ysum_average) is calculated from an integration value of the luminance of the divided blocks with the weighting factor (weight[n]) using the following Equation 1 and used as a fixed target value (in general, a value when the subject appears in gray color as having a reflectivity of 18%).
  Ysum_average  =            ∑              n        =        0                    N        =        63              ⁢                  (                              sum            ⁡                          [              n              ]                                ×                      weight            ⁡                          [              n              ]                                      )            /                        ∑                      n            =            0                                N            =            63                          ⁢                  weight          ⁡                      [            n            ]                              
The exposure control through the average photometry or multi photometry is suitable for achieving exposure corresponding to the brightness throughout the subject. It is, however, impractical to determine the optimum exposure over different phases of the subject. In particular, a difference in brightness between the main subject and its surrounding area will hardly be overcome. An attempt for eliminating such a drawback has been proposed by the same applicant (see Jpn. Pat. Appln. KOKAI Publication No. 2003-8986 (Pat. Document 1)).
The attempt starts with providing as a reference image a subject image taken at the monitor through mode (prior to photographing) using AE by center-weighted photometry and acquiring a histogram indicative of distribution of luminance components in each of a primary region where the subject is supposed to be present and the entire area of the reference image. An exposure compensation possible amount (A) at the positive side indicative of a range in which no white flickers occur in the primary region even with exposure control is obtained on the basis of the histogram in the primary region. Also, a required exposure compensation amount (B) determined to be needed for obtaining proper exposure is calculated on the basis of the histogram in the entire area of the reference image. The exposure control is then carried out based on the required exposure compensation amount (B) with limitation of the exposure compensation possible amount (A). When the exposure compensation possible amount (A) is a negative value, the exposure control action is carried out based on the exposure compensation possible amount (A) but not the required exposure compensation amount (B). According to such a method, the exposure control can be carried out in response to the brightness throughout the image in a range in which the main subject (in the primary region) remains free from white flickers.
However, although the above-described exposure control method is capable of attenuating the occurrence of white flickers on the main subject in its primary region, the image will appear dimmer than the actual view when the center area of the subject is remarkably bright, or more glaring than the actual view when the center area of the subject is remarkably dark because the required exposure compensation amount (B) as a reference for the exposure control is a compensation value based on an EV value corresponding to the brightness of the subject when the reference image is simply taken.
It is hence an object of the present invention, in view of the foregoing drawback, to provide an image pickup apparatus and an exposure control method where the brightness throughout a taken image is controlled to a level closer to the actual level while remaining at a desired level in a predetermined region of the taken image.